Grammar. Just say the word and shivers of aversion travel down the spines of all within hearing distance. Nouns and verbs? Adjectives and adverbs? Is it really necessary to be aware of all these things in order to write a paragraph? Will the reader not be able to work around the tiniest little typo? No. Or, rather, they shouldn’t have to. The reader’s job is to absorb the information: it’s the writer’s job to do the rest of the work.
Why do we bother with grammar? Why not just communicate with words? Because you can’t have one without the other. Letters are put together to make words, words are put together to make sentences, and grammar is the glue that holds it all together. If you don’t use the glue properly – or if you use the wrong glue – all it'll falls well then a parts. Obviously, grammar is entirely necessary, or else you’d be able to understand the last part of that sentence. While most writers are not likely to write anything as catastrophic as that, it’s clear that attention to grammar is important. (It’s okay, though: you don’t have to like the grammar.)